1. Field of the Invention
The present invention is in the field of laser light wave radar, or light detection and ranging (LIDAR), observation of an atmospheric turbulence feature of interest so that background noise may be removed and the signals of interest amplified and observed.
2. Description of the Prior Art
There is no previous method of doing background scattering removal so that the signal of interest may be extracted from the background, and amplified. However, one way it can be done that has been experimentally tried in a laboratory setting is to send out two laser pulses, one pulse when the turbulence is present and the second pulse when the turbulence has changed, or is absent. One return pulse is subtracted from the other return pulse, thus removing the background scattering from the radar return.
There are disadvantages in using this method of subtracting one pulse from the other in that two LIDAR pulses are required for each set of data. The result is that either twice as much time is required to obtain a set of data, or twice the laser pulse repetition rate is required. If the laser pulse repetition rate is doubled, greater physical demands are placed on the LIDAR and the probability of detection are also increased. The two LIDAR pulses must also be sufficiently separated in time for the backscattering to be distinguishable and the backscattering from at least one of the pulses must be stored for later subtracting. Consequently, the background scattering may change materially between the time of sending the two pulses, leading to effects that may obscure the desired signal. Further, a relatively long delay is necessary between LIDAR pulses when long ranges of the atmosphere are being sampled. In the case of where twice as much time is required to obtain a set of data, not only is waveform storage at high data rates required, but the resolution in amplitude of currently available waveform digitizers is not enough to record weak signals of, say, less than 10.sup.-3 times the background without first removing the ambient 1/R.sup.2 background. Since range must be resolved to at least 5 meters in order to recover the atmospheric turbulence signal, a time resolution of 15 nanoseconds or better is required in storing the signal. This requirement places a severe demand on the recording equipment.
With the use of LIDAR it is sometimes necessary to remove the background scattering in order to recover the desired signal. The backscattering of the LIDAR pulse by the homogeneous atmosphere can be, for example, one thousand times larger than the modulation of the atmospheric backscattering due to turbulence and falls off with range as 1/R.sup.2. In order to amplify the backscattering one thousand or more times to detect the turbulence, it is necessary to remove the 1/R.sup.2 variation of the backscatter so that the signal is effectively flat, i.e. constant with range.